do I need to replace the dist gear when I change to a new cam?

You shouldn't have to unless it's a billet roller cam.

If it is still the original steel GM gear, it should be OK forever. If it is an aftermarket gear, I'd replace it.

Its a MSD dist. and a solid Comp Cam

In my experience it depends on how much time you have on it & how many cams it's been used with. I ran a Mallory Unilite for about 25 years & the gear lived through 2 cam changes but it let go right after I swapped in my current Comp solid flat tappet cam. It's an uncommon failure but I figure there must be some slight variances on cam gears that can create different wear patterns. It wasn't that the gear had alot of miles on it since I use my Z28 sparingly, it just sat on top of 3 different cams.